Pathology of the Blood-Retinal Barrier

Abstract

The primary sites of the blood-retinal barrier are the endothelium of the retinal capillary and the retinal pigment epithelium (RPE). Yet, these two types of cells are very different. Embryologically, the endothelium of the retinal capillary is of mesodermal origin, but the RPE derives from neuroectoderm. Structurally, the retinal capillary is of the continuous type.1 The apices of the endothelial cells of the retinal capillary are in direct contact with the bloodstream, and the villi of the endothelial cells are believed to be specialized for transport function. Endothelial cells are joined to each other by a zonula occludens type of cell junction. In contrast, the RPE is not in direct contact with the bloodstream. Nutrients from the choroidal circulation pass through the fenestrated endothelium of the choriocapillaris and Bruch’s membrane to reach the base of the RPE. The basal plasmalemma of the RPE cells develops numerous infoldings to facilitate the transport function. The RPE cells are joined to each other by zonula adherens and zonula occludens types of cell junctions. Physiologically, while the endothelial cells are specialized for transport function, the RPE serves as the metabolic warehouse for the retina and provides support for photoreceptor cells in addition to its function as the blood-retinal barrier for the outer retina.